Describe the bonding and relative inertness of alkanes, their combustion, and the free-radical substitution of alkanes by halogens, including the initiation, propagation and termination steps of the mechanism

What this dot point is asking

SEAB wants you to describe the bonding and relative inertness of alkanes, write equations for their complete and incomplete combustion, and set out the free-radical substitution mechanism with halogens, naming and writing the initiation, propagation and termination steps. The full radical mechanism (with the role of UV) is a guaranteed multi-mark question.

The answer

Bonding and inertness

Alkanes are saturated hydrocarbons with only C-C and C-H single (sigma) bonds. These bonds are strong and, crucially, non-polar (carbon and hydrogen have similar electronegativities). With no polar bonds and no lone pairs, alkanes present no site for an attacking nucleophile or electrophile, so they are relatively unreactive. Their main reactions are combustion and free-radical substitution.

Combustion

Complete combustion (excess oxygen) gives carbon dioxide and water and releases much energy, which is why alkanes are fuels:

Incomplete combustion (limited oxygen) gives carbon monoxide and/or soot (carbon) plus water. Carbon monoxide is a toxic, colourless, odourless gas that binds to haemoglobin more strongly than oxygen, so incomplete combustion is a serious hazard from faulty heaters and engines.

Free-radical substitution

In the presence of ultraviolet light, alkanes react with halogens (e.g. chlorine) by substituting a hydrogen with a halogen. The mechanism has three stages.

Initiation. UV light supplies the energy to break the halogen-halogen bond homolytically (each atom keeps one electron), generating two radicals:

Propagation. A radical reacts to form a product molecule and a new radical, sustaining a chain:

Termination. Two radicals combine, removing radicals and ending the chain:

Why a mixture forms

Because the chlorine radical can abstract a hydrogen at any stage, further substitution gives a mixture of products ($\text{CH}_3\text{Cl}$, $\text{CH}_2\text{Cl}_2$, $\text{CHCl}_3$, $\text{CCl}_4$) and the chain can branch. This lack of selectivity makes free-radical substitution of limited use for clean synthesis.

Examples in context

Example 1. CFCs and the ozone layer. Chlorofluorocarbons release chlorine radicals high in the atmosphere under UV light, and these radicals catalyse the destruction of ozone in a chain reaction analogous to propagation. SEAB uses this environmental example to test radical-chain reasoning beyond simple alkane halogenation.

Example 2. Why fuels need good ventilation. A gas heater run in a poorly ventilated room undergoes incomplete combustion, producing carbon monoxide. Because CO binds haemoglobin more strongly than oxygen, it causes poisoning without warning. This everyday safety context links the combustion equations to a real consequence that exam questions often probe.

Try this

Q1. Explain why alkanes are relatively unreactive. [2 marks]

• Cue. They have only strong, non-polar C-C and C-H sigma bonds, giving no site for nucleophilic or electrophilic attack.

Q2. Write the two propagation steps for the reaction of chlorine with ethane to form chloroethane. [2 marks]

• Cue. $\text{Cl}\cdot + \text{C}_2\text{H}_6 \rightarrow \cdot\text{C}_2\text{H}_5 + \text{HCl}$; $\cdot\text{C}_2\text{H}_5 + \text{Cl}_2 \rightarrow \text{C}_2\text{H}_5\text{Cl} + \text{Cl}\cdot$.

Q3. Write a balanced equation for the complete combustion of butane. [1 mark]

• Cue. $2\text{C}_4\text{H}_{10} + 13\text{O}_2 \rightarrow 8\text{CO}_2 + 10\text{H}_2\text{O}$.